In the industrial world, to allow a lubricant to have high performance, attempts of improving the lubricity of the lubricant by adding various additives thereto have been made. In the case where a bearing or the like is operated under a condition in which the bearing or the like is subjected to a high speed and a high contact pressure, a lubricating film of a grease composition packed in the bearing or the like is liable to be broken. As a result of the breakage of the lubricating film, metal-to-metal contact occurs to cause a problem that heat and frictional wear are increasingly generated. Therefore the grease composition is demanded to show an excellent lubricity when the bearing or the like is operated under a severe condition in which the bearing or the like is subjected to a high speed and a high contact pressure. Mechanical parts such as the bearing, the universal joint, the linear motion device, and the ball screw are operated under the severe condition in which they are subjected to the high speed and the high contact pressure. Thus there is the above-described demand for the grease composition used to lubricate the mechanical parts. To reduce friction when the mechanical parts are operated under the condition in which they are subjected to the high contact pressure, attempts of adding an extreme pressure agent such as an organometallic compound to the grease composition have been made.
The greases for a constant velocity joint which contain a molybdenum compound such as molybdenum dialkyldithiocarbamate (molybdenum dithiocarbamate) or molybdenum dithiophosphate as the organometallic compound are proposed (see patent documents 1 and 2). In order for the constant velocity joint to decreasingly generate vibrations, there is also proposed the grease for the constant velocity joint containing molybdenum dithiocarbamate soluble in base oil, which is combined with molybdenum dithiocarbamate insoluble in the base oil as an extreme pressure agent thereof (see patent document 3).
The grease not containing molybdenum disulfide and containing an organomolybdenum compound and Ca sulfonate is also proposed (see patent document 4). Further, to prevent deterioration of the quality and lubricating performance of the grease composition in using the rolling bearing under a severe condition in which the rolling bearing is subjected to a high temperature, a high load, and a high speed, there is proposed the grease for the rolling bearing containing a microcapsule in which the additive such as a dithiocarbamate is packed (see patent document 5).